1. Field of the Invention
The present invention relates to methods for manufacturing ceramic heating elements.
2. Related Art
Glow plugs can be utilized in any application where a source of intense heat is required for combustion. As such, glow plugs are used as direct combustion initiators in space heaters and industrial furnaces and also as an aid in the initiation of combustion when diesel engines must be started cold. Glow plugs are also used as heaters to initiate reactions in fuel cells and to remove combustible components from exhaust systems.
With regard to the example of diesel engine applications, during starting and particularly in cold weather conditions, fuel droplets are not atomized as finely as they would be at normal running speeds, and much of the heat generated by the combustion process is lost to the cold combustion chamber walls. Consequently, some form of additional heat is necessary to aid the initiation of combustion. A glow plug, located in either the intake manifold or in the combustion chamber, is a popular method to provide added heat energy during cold start conditions.
The maximum temperature reached by a glow plug heating element is dependent on the voltage applied and the resistance properties of the components used. This is usually in the range of 1,000-1,300° C. Materials used in the construction of a glow plug are chosen to withstand the heat, to resist chemical attacks from the products of combustion and to endure the high levels of vibration and thermal cycling produced during the combustion process.
To improve performance, durability and efficiency, new materials are constantly being sought for application within glow plug assemblies. For example, specialty metals and ceramic materials have been introduced into glow plug applications. While providing many benefits, these exotic materials can be difficult to manufacture in high volume production settings. Sometimes, they are not entirely compatible with other materials, resulting in delamination and other problems. Another common problem with specialty materials manifests as tolerance variations when formed in layers resulting from cumbersome and inefficient manufacturing techniques.
Conventional methods for manufacturing ceramic heating elements, such as glow plugs, involve complex manufacturing techniques. For example, one method uses multiple layers of ceramic with different compositions. Each of those layers are built up by sequentially slip casting layers into a porous gypsum mold. The resulting part is removed from the mold and fired to produce a dense ceramic monolithic part. The casting equipment used in this type of manufacturing process is complicated and requires a complex system of pumps and hoses to inject the slurry into the molds. Moreover, the molds require careful preparation and have a very limited lifetime. Other problems exist with this method, including changes in the mold that occur after each use and result in inconsistent layer thicknesses and inconsistent performance in the fired part. Further, conventional methods are limited in their application and thickness of the layers. A thinner layer reduces the stresses associated with thermal expansion differences between layers that can result in delamination of layers during thermal cycling.
Therefore, a need exists for an improved method for manufacturing ceramic heating elements which is less complex than conventional methods and eliminates the difficulties associated with plaster molds and the slurry injection equipment. A method is needed that can build a sequence of thinner layers without compounding variations in the thickness or composition of the layers or increasing stresses associated with thermal expansion differences between the layers. It being understood that high stresses can result in delamination of the layers during the thermal cycling.